1. Field of the Invention
The present invention relates to a surface emitting semiconductor laser used as a source for optical information processing, and more particularly, to a surface emitting semiconductor layer used as an optical source of an optical data storage device and optical communications.
2. Description of the Related Art
Recently, there has been an increased demand for a surface emitting semiconductor laser capable of easily realizing an array of sources in the technical fields of optical communications and optical interconnections. Such a laser is also called vertical-cavity surface-emitting laser diode (VCSEL).
It is known that VCSEL has advantages of low threshold current, small power consumption, easy formation of a circular spot and wafer-level evaluation. On the other hand, it is known that VCSEL has disadvantages due to a small volume of the active region resulting from the low threshold current. More particularly, VCSEL has a high device resistance as much as a few tens to a few hundreds of ohms and difficulty in power up. For example, it is difficult to obtain high power as much as 10 mW by the device alone.
A low-cost multimode optical fiber, which is typically a plastic optical fiber (POF), has been developed, and attention to short-distance optical communications (as short as a few meters to a few hundreds meters) has been occupied. Long-distance optical communications employ the combination of a single-mode optical fiber and an edge-emitting laser diode having a comparatively long wavelength of, for example, 1.31 xcexcm or 1.55 xcexcm. However, these devices are expensive and are unsuitable for local-area communications that require low costs.
The optical source for the multimode optical fiber is required to be less expensive and free of a particular optical system and driving system. The surface emitting laser satisfies these requirements and is one of the good candidates as the optical source for the multimode optical fiber.
A typical VCSEL that is available in the market is of so-called proton implantation type that utilizes a thermal lens effect in which a slight heat-based difference in the refractive index between the current passage region and the peripheral region thereof. The size or diameter of the non-proton-implanted region or the current passage region is set equal to ten to a few tens of micrometers for laser oscillation. However, this has disadvantages in that the light-emission efficiency is low because of weak current narrowing and the threshold current is high. Further, the device characteristics are liable to be degraded because of large heat generation, and the frequency response in the absence of the bias voltage is not good. The proton-implanted type VCSEL is called gain waveguide structure in structural categorization.
Selective oxidization VCSEL has an intentionally formed distribution of the refractive index for optical confinement and is therefore categorized into the refractive index waveguide structure. In the selective oxidization VCSEL, part of a semiconductor multilayer reflection layer in the vicinity of the active region is selectively oxidized to thus define a refractive index waveguide path. The region thus defined has the strong optical confinement effect, and therefore realizes high efficiency and low threshold current. In addition, it is possible to easily fabricate a VCSEL having a good frequency response in which the modulation bandwidth (3 dB down cutoff frequency) ranges from a few to a few tens of GHz.
Although VCSEL has many advantages mentioned above, various high-order oscillation modes may occur by increasing the diameter of the light emitting portion over 10 xcexcm for the purpose of power up, and exhibits a so-called multimode oscillation characteristic. If the spectral width spreads over the given range in the multimode oscillation, increased attenuation of light develops due to the mode dispersion characteristic of the optical fiber. Also, change of the amount of injection current or ambient temperature may cause lateral mode instability of mode, so that mode dispersion noise is liable to take place. Furthermore, increasing of beam divergence angle may degrade the coupling efficiency with the optical fiber.
The mode characteristic may be improved by a method of narrowing the diameter of the light emission region (typically smaller than 4 xcexcm) and restricting the oscillation to only the fundamental (zeroth-order) transverse mode of the smallest order. However, the method encounters a problem such that the device resistance is high and power up is thus unexpected.
It is required to provide tradeoff solving means for stabilizing the transverse mode that is essential to prevent attenuation in optical coupling with the optical fiber, reducing the beam divergence angle and resistance, and powering up without losing the features of the selective oxidization VCSEL of high emission efficiency and good frequency response.
There are many proposed VCSELs intended to control the oscillation mode. For example, U.S. Pat. No. 5,940,422 describes a VCSEL with a mode control using two regions having different film thicknesses. This proposal defines a film thickness equal to xc2xc between the oscillation facilitating region and oscillation suppressing region.
U.S. Pat. No. 5,963,576 discloses a VCSEL having a ring-shaped waveguide path. A circular recess is formed on the top surface of a post, and a ring-shaped light emission region is provided around the recess.
Japanese Unexamined Patent Publication No. 2001-210908 discloses a single-transverse-mode semiconductor laser having a current supply path having a diameter of 10 xcexcm or greater and making a light emission window smaller than the diameter of the current supply path.
However, none of the proposals meet the aforementioned requirements of the optical source for the multimode optical fiber. That is, a laser device with a small beam divergence angle, low resistance, high power, high efficiency and high frequency response has not yet been realized while the stability of the transverse mode is maintained.
The present invention has been made in view of the above circumstances and provides a surface emitting semiconductor laser in which the above disadvantages are eliminated.
According to an aspect of the present invention, the surface emitting semiconductor laser has: a substrate on which a resonator is formed, the resonator including a lower reflection mirror, an active region, and an upper reflection mirror; a metal layer that is provided on the upper reflection mirror and has a first aperture that defines an outgoing region of laser light generated in the active region; and an optical confinement region that is provided between the metal layer and the lower reflection mirror and has a second aperture that defines a light emission region of the laser light, the second aperture having a diameter equal to or greater than 12 xcexcm, and the first aperture having a diameter that is 1 to 5 xcexcm smaller than that of the second aperture, the laser light emitted from the light emission region having a multimode including multiple orders selected within a predetermined wavelength range.
According to another aspect of the present invention, the surface emitting semiconductor laser has: a substrate; a first semiconductor reflection layer of a first conduction type provided on the substrate; an active region for generating laser light provided on the first semiconductor reflection layer; a second semiconductor reflection layer of a second conduction type provided on the active region; a metal layer that is provided on the second semiconductor reflection layer and has an aperture via which the laser light goes out; and a current narrowing region that is interposed between the first and second reflection layers and a conductive portion surrounded by a high-resistance portion, the aperture of the metal layer being smaller than the conductive portion of the current narrowing region, the aperture of the metal layer and the conductive portion of the current narrowing region having sizes that allows the laser light of multimode emitted via the aperture of the metal layer, the multimode containing, in addition to an order of interest, at most two orders other than the order of interest generating a light output larger than those of other orders within a range from a maximum output level of laser light of the order of interest to a level that is 20 dB lower than the maximum output level.
According to yet another aspect of the present invention, the surface emitting semiconductor laser has: a substrate; a first semiconductor reflection layer of a first conduction type provided on the substrate; an active region that is provided on the first semiconductor reflection layer and generates laser light; a second semiconductor reflection layer of a second conduction type provided on the active region; a metal layer that is provided on the second semiconductor reflection layer and has an aperture via which the laser light goes out; and a current narrowing region that is provided between the first and second semiconductor reflection layers and has a conductive portion surrounded by a high-resistance portion, the conductive portion having a diameter equal to or larger than 12 xcexcm, the aperture of the metal layer having a diameter that is at least 1 xcexcm smaller than that of the conductive portion and allowing the laser light of multimode.